Method of forming dimensional holes in the wankel rotor housing electroform

ABSTRACT

An improved process is disclosed for electrodepositing a heavy functional coating onto a complex shaped mandrel with integrally sharply defined openings or separations in the coating. The mandrel serves as a substitute for the final substrate to which the coating is subsequently bonded. The insert may be formed as a solid inert plug severed from circular stock having a curvature identical to area of the mandrel to which the insert is attached. After electrolysis of the insert carrying mandrel, the coating is stripped and insert removed from the coating. The coating with at least one pre-shaped opening is placed in a die-cast machine for casting a substrate therearound having a channel in communication with the opening. The inert plug may have a conductive paint applied to certain surfaces thereof to promote a flared electrodeposited collar about the opening; the collar facilitates good interlock with the cast substrate. Alternatively, the insert may be of a material effective to serve as a permanent part of the coating, the insert being a section of a tube having desirable thermal and wear characteristics.

United States Patent [191 Donakowski et al.

1 Jan. 14, 1975 Morgan, both of Dearborn Heights; Karl Roemming, Detroit, all of Mich.

[73] Assignee: Ford Motor Company, Dearborn,

Mich.

[221 Filed: Nov. 5, 1973 21 Appl. No.: 413,156

[52] US. Cl 164/101, 29/527.3, 204/11,

204/24, 204/25, 164/113, 164/75, 164/76 [51] Int. Cl B22d 19/00, B22d 19/08 [58] Field of Search 29/5271, 527.2, 527.3,

3,628,237 12/1971 Ziegler 29/5276 3,640,799 2/1972 Stephen et al. 204/38 B 3,797,101 3/1974 Bauer 29/5273 Primary ExaminerFrancis S. Husar Assistant Examiner-V. K. Rising Attorney, Agent, or FirmJoseph W. Malleck; Keith L. Zerschling [5 7] ABSTRACT An improved process is disclosed for electrodepositing a heavy functional coating onto a complex shaped mandrel with integrally sharply defined openings or separations in the coating. The mandrel serves as a substitute for the final substrate to which the coating is subsequently bonded. The insert may be formed as a solid inert plug severed from circular stock having a curvature identical to area of the mandrel to which the insert is attached. After electrolysis of the insert carrying mandrel, the coating is stripped and insert removed from the coating. The coating with at least one pre-shaped opening is placed in a die-cast machine for casting a substrate therearound having a channel in communication with the opening. The inert plug may have a conductive paint applied to certain surfaces thereof to promote a flared electrodeposited collar about the opening; the collar facilitates good interlock with the cast substrate. Alternatively, the insert may be of a material effective to serve as a permanent part of the coating, the insert being a section of a tube having desirable thermal and wear characteristics.

9 Claims, 1 Drawing Figure METHOD OF FORMING DIMENSIONAL HOLES IN THE WANKEL ROTOR HOUSING ELECTROFORM BACKGROUND OF THE INVENTION Electrolytic coatings have been formed as liners for transfer to product requiring a wear resistant surface, such liners have not contained openings which are required as spark plug holes, gas intake and exhaust port holes in a product such as the rotor housing of rotary internal combustion engine of the peripheral ported type. Presently, such openings are machined as an independent operation requiring costly tooling and man power. The edges or shoulders of such machined openings constitute critical wear areas of the transfer coating which endure special separating forces during engine operation and endure considerable wear from the plurality of apex seals guided to rub continuously against said transfer coating, traversing the shoulders and edges of such openings. The cyclic variation in heat at such openings along with the constant rubbing action of the apex seals may contribute to a high rate of deterioration, such as delamination.

SUMMARY OF THE INVENTION The primary object of this invention is to provide a more economical method of fabricating a rotor housing for a rotary internal combustion engine, the method employing a transfer coating technique wherein the coating has pre-formed openings therein.

Still another object is to provide a method for improving the joint between an electrolytically deposited transfer coating and a cast substrate therearound, the cast substrate having channels which are in communication with defined openings in the transfer coating.

Features pursuant to the above objects comprise the use of inert inserts preshaped to attach to a predetermined location on the mandrel for precisely defining an opening a coating deposited on the mandrel. The insert may have a conductive paint applied to the edges of the insert which promotes a flared collar about the insert when depositing the coating and which collar promotes better interlock with a substrate cast therearound.

SUMMARY OF THE DRAWING FIG. 1 is a schematic presentation of the sequence of a preferred method for carrying out the present invention, the sequence having substantially four fundamental steps.

DETAILED DESCRIPTION A preferred method for carrying out the present invention, is schematically illustrated in FIG. 1 and essentially comprises (Step 1) both forming a mandrel having an electrolytically conductive surface 11 shaped as an epitrochoid and forming an insert 12 (taken from a contoured stock 21) which is effective to be attached to one local area 13 of the surface of said mandrel. Next, (Step 2), the insert 12 is assembled fastened to the local area of said mandrel surface by suitable means 14 to form a water-tight seal therewith. In Step 3, the assembly is then inserted within an electroplating means 15 where a suitable composite transplant coating 16 of nickel and silicon carbide is deposited upon the mandrel surface generally to the thickness of said insert; the electrodeposited functional coating 16 is then removed or stripped from said mandrel 10 after the fastening means 14 is dissassembled. Means 14 has a fastener 17 which is first removed from said insert [2 and the threaded opening 18 in the mandrel. The insert 12 is simply punched from'said transferred coating [6 to leave a pre-formed liner having a pre-shaped opening 19 therein. Lastly (as in Step 4), the pre-shaped liner or transfer coating 16 is inserted in a die-east machine 20 having a cavity 22 with the transfer mated upon another mandrel 23 in the machine. A substrate, such as aluminum, is cast about the coating with a suitable channel in communication with the opening 19. Subsequent machining removing a small amount ofthe liner surface may be carried out to produce a predeter mined smooth surface.

Forming Mandrel and Insert (Step l) The mandrel 10 is prepared from a suitable core material capable of being machined to a very exact complex configuration, such as the epitrochoid surface [1. An epitrochoid configuration 24 is typically required by the internal wall ofa rotor housing for a rotary inter nal combustion engine. The epitrochoid surface ll must be a mirror image of the resultant epitrochoid configuration 24 to be structured on the rotor housing of the engine. A suitable material for the mandrel is a chromium bearing steel having a chromium content in the range of 3-25 percent. The chromium content en ables the material to be passivated thereby facilitating nonadhesion between the mandrel andthe material to be deposited thereover. In addition, the mandrel may be tapered in a direction (such as 25) from one side to the other to faciliate stripping subsequent or during Step 3 of the process. The surface must have a surface roughness of 4l2 r.m.s. which may be imparted by machining and polishing.

The insert 12 may be preferably defined as a polypropylene plug which is machined to a specified dimension. The sides 12a must be tapered toward the center of curvature and have one face 26 machined with the same radius as the radius of location 13 on the mandrel surface to which it is to be attached. A threaded hole 18 is tapped through the central portion of area 13 for latter reception of a fastener 17. The machining of a pre-shaped plug can be facilitated by cutting the plug from a circular stock 21 having the entire edges 21a thereof tapered and the internal circular face of an appropriate radius to mate with the location 13 of the mandrel; in this way, the plug need only be cut from the stock with tapered transverse edges to complete its formation.

Assembly (Step 2) A thread opening 27 is tapped through plug 12. Plug 12 is then secured to the mandrel 10 with its contoured face 26 secured by means 14 against the location 13 on surface 16. Means 14 comprises a threaded fastener 17, such as a machine screw, and threaded opening 18 in the mandrel. The contoured face of the plug is caused to seat against the mandrel area 13 in a water-tight manner. In the assembled condition, the insert 12 extends about the surface 11, a distance 30 generally equivalent to thickness of the transfer coating to be de posited. The insert distance 30 may preferably be greater than the deposited coating since it will be re moved prior to die-casting. The mandrel then may be prepared for electroplating by being placed in a fixture (not shown) and immersed in hot water in the range at l180F for about 10 minutes to passivate the mandrel surface.

Electroplating and Stripping (Step 3) The assembled mandrel and insert is wiped with a solvent and placed in a plating harness (not shown). The harness may support several mandrels in a vertically stacked arrangement separated by inert spacers. The harnessed mandrels may be given a passivation treatment at this time or as indicated in connection with Step 2, immediately after 2 before harnessing.

Electrolized for 2 hours to produce an electroform with a minimum thickness of 0.02 inches. Current density, first 5 minutes at amps with 2.6 volts, temperature 160F, the next 10 minutes at amps with 4 volts, the last 105 minutes at 45 amps with 8 volts.

The harnessed mandrels are then placed in an electroplating means 15 to receive a coating which preferably comprises a dispersion coating of nickel with uniformly distributed silicon carbide particles. A preferred electrolyte to carry out such plating may comprise an aqueous solution having 300-350 grams/liter of nickel sulfamate, about 120 grams/liter of silicon carbide, 45 grams/liter of boric acid, about 17 grams/liter of nickel chloride and about 2.5 grams/liter of a stress reliever (saccharin).

The harnessed mandrels are constituted as a cathode for the electroplating function and a suitable anode assembly containing nickel surrounds the mandrel in a conforming relationship. A potential is applied between the anode and cathode effective to maintain current density of at least 200 amps/ft with the bath at a temperature of at least 160F.

The electrolysis is carried out typically for a period of 2 hours in order to deposit a functional coating at 0.030 inch thick. This may be separated into three periods; for example, in the first 5 minutes the current density impressed may be low with only 2.6 voltage. In the next 10 minutes, the voltage may be increased to 4.0 and the amperage doubled. in the remaining 105 minutes the voltage must be 810 and the amperage 45. During the latter period, high current density is impressed.

After deposition, the harnessed coated mandrel is removed from the electrolyte, the stainless steel screw 17 is removed, and the coating 16 stripped by slipping the deposit off sideways from the mandrel which may have a taper to facilitate such stripping. Only light forces are required to effect such stripping. The insert 12 is also pressed out in the direction of the arrows indicated in FIG. 1; this again is facilitated by the taper of the edges 12a of the insert. The resulting stripped coating has a neatly and accurately defined opening and is immediately ready for transfer to the die-cast machine of Step 4. Such opening is inherently accurately located relative to the entire coating and has a dimensionally accurate sizing. Attempting to machine such an opening can never result in the low reject rate that this process affords. Limited frequency of breakage of the opening edge or slight dimensional deviation of the opening accuracy, produces a reject rate over large volume production which can be undesirable.

Die-Casting and Machining The transfer coating 16 is placed in a die-cast machine having a suitable cavity 22 which defines the body of the rotor housing. The coating 16 is placed about another mandrel 23 in the cavity. Suitable coring 40 is employed to define a channel in the substrate which will have a throat 41 in communication with and identical in size to the dimensionally accurate opening 19 of the transfer coating.

Some desirable alternatives may be employed with the above process. The insert 12 may have a conductive paint applied to thetapered edges thereof; if the thickness of the insert is increased somewhat substan tially beyond the intended thickness of coating (forexample twice the thickness), there will be produced an electrodeposited flared collar about opening when the insert is removed. This results from the fact that the conductive paint extends the cathodic surface away from the mandrel surface encouraging an electrolytic deposit about the upstanding edges of the insert.

Thus, when the coating 16 is wedded to the substrate during casting, the flared collar will offer a superior in terlock with the substrate tending to resist delamination at the opening edges under the severe thermal cycausing a flared collar about the section; the inner opening of the section should be masked to prevent electrodeposition in this area. The tube section will be retained as a permanent part of the transfer coating and offers the introduction of special metallurgical materials about the opening shoulder while retaining the ease of fabricating dimensionally accurate openings in a transfer coating.

We claim as our invention:

1. A method of fabricating a rotor housingfor a rotary internal combustion engine, comprising:

a. forming a mandrel having an electrolytically conductive surface, and forming an insert having a thickness greater than a subsequent electrolytic de posit on said mandrel, said insert having a face formed as a mirror image of one area of said mandrel surface,

b. assembling said insert to said mandrel at said one area and securing said assembly to effect a watertype seal therebetween,

c. electrodepositing a wear-resistant metal-carbide composite onto said mandrel surface having a thickness substantially equivalent to but less than said insert,

(1. removing said securement retaining said insert to said mandrel, stripping said electrolytic transfer coating from said mandrel, and removing the insert from the transfer coating, and

e. placing said stripped transfer coating in a die-cast machine and casting a substrate metal therearound to complete said rotor housing, said housing having at least one channel with a throat dimensionally equal to and in communication with the opening of said transfer coating.

2. A method as in claim 1, in which said substrate is aluminum and said composite of said transfer coating is a dispersion of silicon carbide in nickel.

3. The method as in claim 1, in which said insert has tapered edges aligned in a direction so that the insert may be easily removed from said transfer coating by punching.

4. A method as in claim 1, in which said mandrel defining said conductive surface is comprised of a chromium-bearing steel having a chromium content in the range of 3-25 percent, and said insert is formed as a solid plug constituted of polypropylene.

5. A method as in claim 1, in which said insert has a conductive paint applied to its edges whereby as electrodeposited flared collar is created about said opening effective to provide an improved joint between the transfer coating and substrate at the edges of said openmg.

6. A method as in claim 1, in which said securement is provided a tapped threaded opening centralized in said area to which said insert is to be attached, and a threaded fastener extending through a portion of said insert into said threaded opening clamp said insert to said mandrel for creating a water-tight seal, said threaded fastener being removed first during said stripping step.

7. The method as in claim 2, in which said electroplating step is carried out with said mandrel constituted as a cathode and an anode constituted of nickel, the electrolyte for said electroplating operation being comprised of an aqueous acidic solution substantially consisting of at least 600 grams/liter of nickel sulfamate,

about 45 grams/liter of boric acid (H BO about 2.5 grams/liter of stress reliever and about 17 grams/liter of nickel chloride.

8. The method as in claim 7, in which the current density imposed between said anode and cathode is stepped during the time period of said electroplating, the first step having current density substantially below 200 amps/ft with a bath temperature at about l6()F, the second step having a current density of in excess of 200 amps/ft? 9. The method as in claim 1, in which said insert is comprised of a section of an electrically conductive tube, section having a thickness along the axis of said tube which is greater than the deposited coating, said conductive insert being maintained as an integral part of said transfer coating and is not stripped from said transfer coating before or after die-casting the substrate therearound. 

1. A method of fabricating a rotor housing for a rotary internal combustion engine, comprising: a. forming a mandrel having an electrolytically conductive surface, and forming an insert having a thickness greater than a subsequent electrolytic deposit on said mandrel, said insert having a face formed as a mirror image of one area of said mandrel surface, b. assembling said insert to said mandrel at said one area and securing said assembly to effect a water-type seal therebetween, c. electrodepositing a wear-resistant metal-carbide composite onto said mandrel surface having a thickness substantially equivalent to but less than said insert, d. removing said securement retaining said insert to said mandrel, stripping said electrolytic transfer coating from said mandrel, and removing the insert from the transfer coating, and e. placing said stripped transfer coating in a die-cast machine and casting a substrate metal therearound to complete said rotor housing, said housing having at least one channel with a throat dimensionally equal to and in communication with the opening of said transfer coating.
 2. A method as in claim 1, in which said substrate is aluminum and said composite of said transfer coating is a dispersion of silicon carbide in nickel.
 3. The method as in claim 1, in which said insert has tapered edges aligned in a direction so that the insert may be easily removed from said transfer coating by punching.
 4. A method as in claim 1, in which said mandrel defining said conductive surface is comprised of a chromium-bearing steel having a chromium content in the range of 3-25 percent, and said insert is formed as a solid plug constituted of polypropylene.
 5. A method as in claim 1, in which said insert has a conductive paint applied to its edges whereby as electrodeposited flared collar is created about said opening effective to provide an improved joint between the transfer coating and substrate at the edges of said opening.
 6. A method as in claim 1, in which said securement is provided a tapped threaded opening centralized in said area to which said insert is to be attached, and a threaded fastener extending through a portion of said insert into said threaded opening clamp said insert to said mandrel for creating a water-tight seal, said threaded fastener being removed first during said stripping step.
 7. The method as in claim 2, in which said electroplating step is carried out with said mandrel constituted as a cathode and an anode constituted of nickel, the electrolyte for said electroplating operation being comprised of an aqueous acidic solution substantially consisting of at least 600 grams/liter of nickel sulfamate, about 45 grams/liter of boric acid (H3BO3), about 2.5 grams/liter of stress reliever and about 17 grams/liter of nickel chloride.
 8. The method as in claim 7, in which the current density imposed between said anode and cathode is stepped during the time period of said electroplating, the first step having current density substantially below 200 amps/ft2 with a bath temperature at about 160*F, the second step having a current density of in excess of 200 amps/ft2.
 9. The method as in claim 1, in which said insert is comprised of a section of an electrically conductive tube, section having a thickness along the axis of said tube which is greater than the deposited coating, said conductive insert being maintained as an integral part of said transfer coating and is not stripped from said transfer coating before or after die-casting the substrate therearound. 